This invention relates to a fermented milk product in which the increase in acidity in the low pH region and during storage at lower temperatures is inhibited, and novel lactic acid bacteria of the genus Lactobacillus that may be employed for the preparation of such fermented milk product.
The lactic acid bacteria of the genus Lactobacillus have long been known as a representative starter for preparation of fermented milk. The lactic acid bacteria of rod shape, such as the genus Lactobacillus produces lactic acid of higher acidity than that of the lactic acid bacteria of coccus such as the genus Lactococcus or the genos Stretococcus, while being frequently higher in its protease activity of decomposing milk protein. In particular, Lactobacillus helveticus exhibits strong protein-decomposing activity. The peptide generated by decomposition of milk protein by its extracellular protease has been reported to exhibit the inhibition activity against angiotensin converting enzyme (Hereinafter referred to as ACE) which is a substance responsible for increase in blood pressure. Similar activity may be noticed with the fermented milk by the Lactobacillus helveticus. These ACE inhibitory peptides have been confirmed to exhibit the activity in lowering the blood pressure with spontaneously hypertensive rats (SHR), as reported by Nakamura, Y. et al NIPPON NOGEI KAGAKU KAISHI, 67, 289, 1993.
However, Lactobacillus helveticus produces a large quantity of lactic acid during milk fermentation and hence is lowered significantly in pH. Besides, the acidity of lactic acid tends to be increased during storage at low temperature. Consequently, Lactobacillus helveticus is difficultly used for the preparation of yogurt, etc., such that it is scarcely used at present for yogurt-like fermented milk products.
In general, the increase in acidity during storage following the preparation of yogurt poses a serious problem. For suppressing such increase in acidity, various trials have been made such as inprovement of strains by mutation. However, desired mutant strains have not been produced because mutant strains low in growth activity tend to be separated.
Adenosine triphosphatase (Hereinafter referred to as ATPase) has so far been confirmed to be taking part in intracellular pH adjustment in the microorganisms. Although the extracellular pH of lactic acid bacteria is in an acidic range by the production of lactic acid as a final metabolite in the fermentation of lactic acid bacteria, the intracellular pH of the lactic acid bacteria is maintained in a neutral range by the action of ATPase. On the other hand, ATPase tends to be produced in an increasingly larger quantity with increase in the difference between the intracellular pH and the extracellular pH. Thus, it has been confirmed that, under the conditions in which such cell-wall barrier is destroyed in the acidic medium to eliminate the protonic concentration gradient, the microorganism cells cannot be proliferated (The Journal of Biological Chemistry, 261, 2.627-630, 1986, by Hiroshi Kobayashi, Takeshi Suzuki and Tsutomu Umemoto; and The Journal of Dairy Science, 74, 747-751, 1991, by Nancy L-Nannen and Robert W. Hutkins). While, these reports are directed to the relation between the extracellular pH and ATPase activity of the cells, there is no disclosure as to the final pH of a fermented product obtained by fermentation of the microorganisms.
Concerning neomycin resistance, the neomycin-resistant mutant strain Escherichia coli has been known to be devoid of ATPase activity (J. of Bacteriology, 116, 3. 1124-1129, 1973, by Barry P. Rosen). However, there is no disclosure as to lactic acid production.